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Consensus Seasonal Weather Outlook
June, July and August (JJA)
Seasonal Rainfall and Temperature for Sri Lanka
This forecast was prepared using
The prevailing global climate conditions.
Forecasts from different climate models from around the world.
Statistical downscaling of GCM output using CPT
Issued by Centre for Climate Change Studies (CCCS)
And
Research Division
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Prevailing global climate conditions
In April 2020, positive sea surface temperature (SST) anomalies weakened and were near zero
by the end of the month. During the last four weeks, equatorial SSTs were near-to-below normal
across the central and eastern Pacific Ocean, and above normal in the western Pacific Ocean with
the ENSO neutral conditions (Fig 1).
Above-average SSTs weakened across the equatorial Pacific Ocean, and below-average SSTs
emerged in the east-central Pacific during the last four weeks. (Fig. 2).
The overall ocean temperature patterns are consisting with ENSO-neutral. According to the
NOAA, ENSO-neutral is favored through Northern Hemisphere Summer 2020 (~65% chance),
with the chances decreasing through the autumn 2020 (~45% to 55% chance).
The IRI/CPC Official Probabilistic Forecast (Fig 3 A) likely to continue with the probability to
favor ENSO-neutral (Niño-3.4 index between -0.5°C and +0.5°C) through the Northern
Hemisphere Summer 2020, with the possibility of below-average temperatures becoming more
established toward the latter half of the year. It shows the possibility of getting La Niña
conditions during this period. (Climate Prediction Center, USA).
The Indian Ocean Dipole (IOD) is also currently neutral and the latest weekly value to 27 May
was +0.3 °C. There is currently little temperature gradient across the Indian Ocean, which is
consistent with neutral IOD. (Fig 3 B) (Bureau of Meteorology, Australia). Most of modal
outlooks indicate the IOD is likely to become negative over the coming months.
Fig 1: Observed Average sea surface temperature (SST) anomalies (°C)
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Fig 2: Weekly Observed Average sea surface temperature (SST) anomalies (°C)
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(A)
(B)
Fig 3a: ENSO forecast from Climate Prediction Center (CPC)/ IRI Forecast (A) and IOD
forecast from Australian Bureau of Meteorology (B).
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Impacts of negative IOD on monthly rainfall anomaly during July and August
Fig 3b: Median Based Composite maps of Monthly Rainfall during July (A) and August (B) during IOD
years (Hapuarachchi et al 2018)
Previous studies conducted by the Department of Meteorology, identified that the probability of
getting above normal rainfall in the Southwestern parts of Sri Lanka, and some parts of
Batticaloa and Ampara districts in July is high (Fig 3b (A)) during negative IOD condition. But
in the month of August, is showing the probability for above normal is very low in the
Southwestern quarter, although the probability is high in the Eastern, Northern and Central parts
of Sri Lanka (Fig 3b (B)).
(a) Forecasts from different climate models from around the world.
(a.1) From June 2020 to August (JJA) 2020 season
Figure 4 shows the probabilistic multi model ensemble forecast which prepared by using
dynamical models from 10 Global Producing Centers (GPC) for JJA season. According to that
above normal rainfall can be expected over the country for JJA season. 2020.
(A) July (B) August
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Fig 4: Probabilistic multi model ensemble forecast for JJA using dynamical models from 10
WMO global producing centers (GPC).
Figure 5 depicts individual forecasts provided by same GPC centers for the JJA season. Out of
10 GPC individual models, 5 predicted above normal rainfall, 1 predicted above to near normal
rainfall and there is no clear signal over the Island from 4 models during the JJA season.
Accordingly, it can be expected near to above normal rainfall over the country during JJA 2020
season.
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Fig 5: Individual forecasts for JJA 2020 season by dynamical models from 10 WMO global
producing centers (GPC).
(a.2) Monthly Forecast for June, July and August 2020
Figure 6 shows the probabilistic multi model ensemble forecast, which is prepared by using
dynamical models from 10 global producing centers (GPC), for the months of June, July and
August 2020. According to that, above normal rainfall can be expected over most parts of the
Island during the months of June and July. While there is no clear signal over the country except
Northern parts during the month of August 2020.
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Fig 6: Probabilistic multi model ensemble forecast for June (left), July (middle) and August 2020
(right) using dynamical models from 10 WMO global producing centers (GPC).
Fig 7: Individual forecast for June 2020 by dynamical models from 10 WMO global producing
centers (GPC).
Figure 7 shows the 10 monthly forecasts from individual global producing centers (GPC) for
June 2020. Out of 10 GPC forecasts, 6 GPC models predicted above normal rainfall and there is
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no signals from 4 GPC models for the month of June 2020. Accordingly, it can be expected
above normal rainfall over the island during the month of June 2020.
Fig 8: Individual forecast for July 2020 by dynamical models from 10 WMO global producing
centers (GPC).
Figure 8 shows the monthly forecasts from individual global producing centers (GPC) for July
2020. Out of 10 GPC forecasts, 6 GPC models predicted above normal rainfall, while no clear
signal indicated in 4 GPC models for the month of July 2020. Accordingly, above normal rainfall
can be expected over the country during the month of July 2020.
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Fig 9: Individual forecast for August 2020 by dynamical models from 10 WMO global
producing centers (GPC).
Figure 9 shows the monthly forecasts from 10 individual global producing centers (GPC) for
August 2020. Out of 10 GPC forecasts, 4 GPC models indicate above normal rainfall and 1 GPC
model shows below normal rainfall and there is no signal from 5 GPC models for month of
August 2020. Accordingly, there is no signal for August 2020 indicating equal chances of
receiving below normal, near normal and above normal.
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(b) Statistical downscaling of CFSv2 global forecast output
(b.1) Probabilistic Forecast for JJA season 2020 using Climate Predictability tool (CPT)
The following district wise probabilistic rainfall forecasts for the season of JJA 2020 have been
prepared with the multi model ensemble method to downscale ,SST data of CFSv2, CCMS4,
GFDL and ECMWF by using CPT.
The district wise 30 year average rainfalls during JJA season are given in the column 2 of the table
1. Chance (probability) of receiving below/about/above average is given in the columns 3, 4, and 5
respectively in the table1.
Table 1: Probabilistic Rainfall Forecast for JJA season 2020 using CPT
District Average rainfall (mm) –JJA Probability%
Below Normal Above
Colombo 590.1 50 20 30
Kalutara 861.5 55 20 25
Galle 771.0 50 25 25
Matara 604.3 30 25 45
Hambantota 146.9 20 20 60
Ampara 131.5 25 25 50
Batticaloa 141.2 25 25 50
Trincomalee 168.8 30 30 40
Mullaithivu 99.5 35 30 35
Jaffna 71.6 25 30 45
Killinochchi 57.1 25 30 45
Mannar 40.3 30 30 40
Puttalam 98.4 35 30 35
Gampaha 478.5 45 25 30
Kegalle 934.8 50 25 25
Ratnapura 675.1 30 25 45
Monaragala 125.9 20 30 50
Badulla 183.7 20 20 60
Pollonnaruwa 128.9 25 30 45
Vavuniya 109.8 40 30 30
Anuradapura 89.7 25 30 45
Kurunegala 212.4 25 30 45
Matale 163.5 25 30 45
Kandy 497.6 35 30 35
Nuwaraeliya 767.1 30 25 45
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Fig 10: Probabilistic rainfall forecast for June-August 2020 using CPT
According to the CPT (Fig 10 and table 01), above normal rainfalls can be expected in 14
districts out of 25. Below normal rainfall can be expected in Gampaha, Colombo, Kegalle,
Kalutara and Galle Districts. There is no clear signal for Mannar, Vavuniya, Mullativu,
Trincomalee, Puttalam and Kandy districts for JJA season 2020. Therefore equal chances exist of
receiving below normal, about normal or above normal rainfall in Mannar, Vavuniya, Mullativu,
Trincomalee, Puttalam and Kandy districts for JJA Season 2020
.
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(b.2) Probabilistic Forecast for JJA 2020 season using RIMES FOCUS System
Fig 11. Probabilistic rainfall forecast for June-August 2020 using RIMES FOCUS System
Figure 11 depicts the Probabilistic rainfall forecast for climatic zones for JJA 2020 season, which
has been prepared by using RIMES FOCUS System. Accordingly, above normal rainfall is likely
over the Intermediate zone and Dry zone. There is no clear signal indicated for wet zone. It
indicates that there is equal chances for having below, about or above normal rainfall over the
wet zone during JJA season 2020.
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(b.3) Probabilistic Forecast for JJA season 2020 using Climate Information Toolkit
A climate information toolkit, which has been developed by APCC Climate Center (APEC) in
Korea, is used to develop following forecast. It is being used Collection of Dynamic ensemble
seasonal prediction data from National Meteorological and Hydrological Services and research
institutes. This includes 14 operations and the models developed by institutes from 10 countries.
Areal rainfall data are used as input data to ‘’CLIK’’ toolkit and Downscaled to districts level.
SST is selected as predictor for all the models. (APCC, NASA, NCEP, PNU).
Fig 12: CLIK Multi model Ensemble Probabilistic Forecast for JJA season 2020
According to the above model outputs, near normal rainfall can be expected in 13 districts out of
25. Above normal rainfall can be expected Badulla district and below normal rainfall can be
expected in Kandy and NuwaraEliya districts. There is no clear signal for Gampaha, Colombo,
Kegalle, Kalutara, Galle, Matara, Hambantota, Puttalam and Vavuniya districts. It indicates
equal chances of receiving below normal, near normal or above normal rainfall over those
districts during JJA 2020 season.
B - Below Normal
A - Above Normal
N - Near Normal
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Summary : Consensus Seasonal Rainfall outlook for June, July and August
2020 (JJA) Season.
Table 2: SUMMARY of MODEL FORECAST for JJA season for SRI LANKA
SUMMARY of MODEL FORECAST for JJA 2020season for SRI LANKA Season WMO LC
MME
WMO
GPC
CPT FOCUS Impact of
Global
conditions
Final
JJA season
2020
AN AN AN – Most parts except
BN- Gampaha, Kegalle,
Colombo, Kalutara and
Galle
No Signal- Mannar,
Vavuniya, Mullativu,
Trincomalee, Puttalam and
Kandy
AN- Dry
zone and
Intermediat
e zone
No Signal-
Wet zone
Slightly
negative
IOD
conditions
in July and
August
AN
June 2020
No Signal -
Eastern and
Southern parts
AN- elsewhere
AN
AN –Badulla, Monaragala
and Hambantota
BN- Gampaha, Kegalle,
Colombo, Kalutara, Galle,
Mulativu, Vavuniya,
Tricomalee, Batticaloa and
Ampara
No Signal
NIL Near normal
July 2020 No Signal –
Southern parts
AN- elsewhere
AN
- - Slightly
negative
IOD
Above normal in
Sw part and near
normal elsewhere
August 2020 AN- Northern
parts
No Signal –
elsewhere
No
Signal
- - Slightly
negative
IOD
No signal-CP
BN: Below Normal NN: Near Normal AN: Above Normal CP: Climatological
Probability
Consensus Forecast for Sri Lanka
ENSO Neutral conditions are likely to continue during June to August 2020 and IOD is likely to
become slightly negative during July and August 2020.
Considering the prevailing global climate conditions, forecasts from different global climate
models and statistical downscaling of GCM output using CPT, consensus forecast for June to
August 2020 is concluded as follows.
There is a possibility for above normal rainfalls over Sri Lanka for JJA season
There is a chance to receive near normal rainfall over the country during June 2020. Higher
chance is likely to receive above normal rainfalls in Southwestern parts and near normal rainfall
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elsewhere during the month of July 2020. No clear signal over Sri Lanka for the month of
August and it indicates that there is equal chance to receive below, about or above normal
rainfalls during August 2020.
However, the predictability is also limited due to strong day-to-day atmospheric variability
caused by the passage of the synoptic scale systems such as lows and depressions etc. and
intraseasonal Oscillations such as Madden Julian Oscillations (MJO).
Fig 13. Consensus Probabilistic rainfall forecast for June–August 2020
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Probabilistic Temperature Forecast for June–August 2020 (JJA)
The probabilistic Temperature forecast for June, July and August season (JJA) 2020 for Sri
Lanka as given below.
Fig 14: Probabilistic forecast for Maximum Temperatures for JJA season 2020
Fig 14 and Table 3 show the probabilistic forecast for Maximum Temperatures during JJA season
2020.
Accordingly, there is a higher chance of experiencing slightly above the normal Maximum
temperatures in Mannar, Puttalam, Anuradhapura, Trincomalee, Batticaloa, Ampara, Kandy and
Hambantota districts and below normal maximum temperatures in Nuwaraeliya and Rathnapura
districts during JJA season 2020.
The district wise average Maximum Temperatures are given in the column 2 of the table 3 and
the chance (probability) of receiving below/about/above average is given in the columns 3, 4,
and 5 respectively.
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Table 3: probabilistic forecast for Maximum Temperature for JJA season 2020
Fig 15: Probabilistic forecast for Minimum Temperatures for JJA season 2020
District
Average Maximum
Temperature (0C) –
(JJA) Probability %
Below Normal Above
Anuradhapura 33.0 20 30 50 Badulla 30.6 30 35 35 Batticaloa 33.5 20 30 50 Colombo 30.1 35 35 30 Galle 28.7 35 35 30 Hambantota 30.3 20 30 50 Katugastota 28.0 20 30 50 Katunayake 30.5 30 35 35 Mannar 30.8 20 30 50 MahaIlluppallama 32.8 25 30 45 NuwaraEliya 18.7 50 30 20 Pottuvil 31.4 20 30 50 Puttalam 31.5 20 30 50 Ratnapura 30.8 50 30 20 Ratmalana 30.3 35 35 30 Trincomalee 34.5 20 30 50 Vavuniya 33.9 35 30 35 Kurunegala 30.9 30 35 35 Bandarawela 25.8 35 35 30
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Fig 15 and Table 4 provide the probabilistic forecast for Minimum Temperatures during JJA
season 2020.
Accordingly, there is a higher chance of experiencing slightly above the normal minimum
temperatures in Mannar, Vavuniya, Anuradhapura, Kurunegala, Batticaloa, Kandy and
Nuwaraeliya districts and below normal Minimum temperatures in Puttalam, Gampaha,
Colombo, Rathnapura, Galle and Hambantota districts during JJA season 2020.
Table 4: Probabilistic forecast for Minimum Temperatures for JJA season 2020
District
Average Minimum
Temperature (0C) –
(JJA) Probability %
Below Normal Above
Anuradhapura 24.6 20 30 50 Badulla 18.6 30 30 40 Batticaloa 25.3 25 30 45 Colombo 25.3 50 25 25 Galle 24.9 50 25 25 Hambantota 24.8 50 30 20 Katugastota 21.1 20 30 50 Katunayake 24.9 50 30 20 Mannar 26.7 25 25 50 MahaIlluppallama 24.4 20 35 45 NuwaraEliya 12.9 20 30 50 Pottuvil 23.0 35 30 35 Puttalam 25.8 50 30 20 Ratnapura 23.5 50 30 20 Ratmalana 25.0 45 30 25 Trincomalee 25.9 30 35 35 Vavuniya 24.4 25 30 45 Kurunegala 23.8 20 30 50 Bandarawela 16.7 30 35 35